Structure of the Plesiosaurus Macrocephalus. 619 



4th. The superior spinous process*, which is connected, and generally an- 

 chylosed with the distal extremities of the neurapophyses, and forms, in con- 

 junction with those processes, the superior arch of the vertebra. 



5th. An inferior spinous process f, which is connected and commonly an- 

 chylosed with the distal extremities of the hamapophyses, forming-, in con- 

 junction with these, a chevron or v-shaped bone. 



To the category oi autogenous vertebral pieces belong the ribs, which ge- 

 nerally are anchylosed to the other vertebral elements in the cervical, sacral, 

 and caudal vertebrae of the warm-blooded vertebrate classes. 



The propriety of regarding the ribs as vertebral elements is well illus- 

 trated in the Plesiosaurus, in the cervical, sacral, and caudal vertebrte of 

 which they assume the form of, and have been generally described as, trans- 

 verse processes, although they are separate bones. 



These elements bear the same relation to the centrum and its true transverse 

 processes which the spinous processes do to the neur- and haem-apophyses, 

 but they are more rarely anchylosed at their central or proximal extremities. 



The length which the ribs attain need form no objection to their being re- 

 garded as parts of a vertebra, when it is remembered that the spinous pro- 

 cesses, both above and below, are in some fishes longer than the longest ribs 

 in the same skeleton. 



In the system of M. Geoffroy St. Hilaire the nine elements of a vertebra 

 are completed by reckoning the spines of the dermal skeleton which, in fishes, 

 are intercalated or articulated with the neural and htemal spines of the true 

 endo-skeleton as essential elements of a vertebra; and the paraaux, or hcema- 

 pophyses, are described as being developed in length and changed in direc- 

 tion in order to form the vertebral ribs of the thoracic and abdominal legions. 



The vertebrae of the Bird and Ophidian, already alluded to, prove that ver- 

 tebral ribs and inferior laminae or haemapophyses may co-exist, and the com- 

 position of the spine of the Plesiosaurus, especially in the caudal region, 

 well illustrates this fact ; for the costal appendages, which are generally an- 

 chylosed to the other vertebral elements in the cervical, sacral, and caudal 

 regions of the spine of the warm-blooded vertebrate classes, retain their ori- 



words are, in reference to an analogous case, " Donner a un mot connu un sens nouveau est toujours 

 un precede dangereux, et, si Ton avoit besoin d'exprimer une idee nouvelle, il vaudroit encore mieux 

 inventer un nouveau terme, que d'en detourner ainsi un ancien." — Mem. du Mus., tome ix. p. 123. 



* This is regarded by Geoffroy to consist essentially of two lateral moieties, termed epiaux, or 

 epivertebral elements. 



t The hypothesis of Geoffroy necessitates the consideration of this part as being essentially 

 double, forming the cataaiix, or catavertebral elements. 



